1. Field of the Invention
The present invention relates to methods and apparatus for testing material samples. More specifically, the present invention relates to methods and apparatus for determining the cured compressive strength and other characteristics of a cement slurry sample.
2. Description of the Prior Art
Cement is used at different times during the drilling, completion, and repair of wells to bond the well casing pipe into the wellbore, to seal off portions of the subsurface formation from the wellbore, to repair damage in a cased or uncased well, and for various other purposes. Generally, the cement is pumped into the well as a relatively low viscosity slurry. After the cement slurry is in place, it hardens to a solid state. Throughout these various construction and repair procedures, it is important that the cement retain a sufficiently low viscosity as it is being moved into place within the well or formation. It is also necessary that the strength and physical characteristics of the hardened or cured cement be adequate to perform the intended functions in the well.
The curing time and the viscosity change during the curing of cement are dependent upon the materials used in forming the cement slurry, the method used in preparing and injecting the material, and the environmental conditions of the subsurface site. Before undertaking a cement injection procedure, it is essential to know that the pumping and setting characteristics and other physical attributes of the cement to be injected into the well are within design requirements for the specific site.
Initial methods of testing a particular composition of cement required testing viscosity changes of the curing sample over a period of time and destructively testing cured samples of the cement for compression strength. These techniques were superseded by the development of methods and apparatus for evaluating the sample by propagating acoustic energy waves through the uncured sample and evaluating the speed of travel of the waves through the sample to obtain information about the curing time and compressive strength of the cured cement. The background of ultrasonic testing and the devices used in such testing for determining characteristics of structural concrete are described in a paper entitled "Characterization of the Initial, Transitional and Set Properties of Oilwell Cement," Society of Petroleum Engineers paper SPE 36475, 1996. A related publication, SPE 36476, entitled "Analyzing Cements and Completion Gels Using Dynamic Modulus," 1996, describes the theory, design, and operation of an ultrasonic apparatus that measures the dynamic modulus of cements and gels. One of the early systems employed to ultrasonically test cement is described in U.S. Pat. No. 4,259,868, to Rao et al., issued in 1981.
Various sample-holding containers have been employed to hold the cement slurry sample during the performance of sonic tests. One such container, which also permits the sample to be pressurized and heated, is described in U.S. Pat. No. 4,377,087, issued in 1983 to Francois Rodot. The Rodot patent describes a sample-holding device that employs two electro-acoustic transducers that are positioned across from each other to measure the transit time of a sonic signal traveling from one transducer to the other through the sample slurry. The information derived from the test is used to determine the setting and hardening characteristics of the cement sample.
The sample-holding device of the Rodot patent places the transmitting and receiving transducers in intimate contact with the cement slurry sample. A flexible diaphragm covers the top of the slurry sample within the sample chamber. A pressure line on the opposite side of the diaphragm supplies a pressurizing fluid to the sample chamber, which in turn pressurizes the cement sample.
An alternative holder described in the Rodot patent employs oil as a pressurizing medium. During the filling and testing process, the sample-holding apparatus must be maintained in a fixed orientation to prevent the cement sample from escaping the testing chamber.
Both of the sample-holding devices described in the Rodot patent require the oil to surround the electro-acoustic transducers with the oil being in direct contact with the cement sample. Each time a sample is tested, the oil-containing portion of the container must be refilled with oil to ensure uniform pressure in the sample and on the transducers.
Where large pressures are to be applied to the sample, or where the transducer can be adversely affected by large pressure differentials acting across its structure, it becomes critical to protect the transducers from pressure extremes that may occur during the testing procedure. In this regard, leakage past a flexible seal or other failure that prevents proper pressure equalization between the pressured cement and the transducer body can prevent proper operation of the transducers during the testing process and can also severely damage the transducer.
Another danger that becomes increasingly threatening as the testing pressures are increased is that of water leaking into contact with the transducer's electrical conductors. The existence of such leakage can prevent the transducer from functioning properly and can produce faulty readings in the equipment that analyzes the signals transmitted from the transducer.
In sonic testing procedures, where received acoustic energy waves are analyzed to determine characteristics of the medium through which the waves travel, knowledge about the wave path becomes critically important. If the generating transducer produces acoustic energy waves that radiate from different areas of the transducer, the received signal may be a combination of signals traveling over different paths. The result is a complex signal that is difficult to correctly analyze. For purposes of cement sample analysis, optimum information is obtained from a received signal that is generated from a single, known surface and travels over a known, straight-line path.